Whenever fluids are forced through conduits such as ducts, turbulence occurs because of the nature of circulating equipment and non-uniformity of the conduit itself. Associated with this turbulence is a non-uniformity of the velocity across any selected arbitrary plane in the conduit. Generally the velocity is greater near the center of the conduit; however, physical structures (obstructions, shapes, etc.) of the conduit can shift the region of higher velocities away from the center. The velocity is generally the lowest near the walls of the conduit due, at least in part, to the friction of the fluid along those walls.
The circulation of air in heating, ventilating and air conditioning systems is typical of the fluid systems wherein turbulence and non-uniform velocity distribution occur. In many applications such matters are not detrimental unless excessive resistance to flow occurs or turbulence causes excessive vibration. However, in applications where accurate velocity or volume flows are to be determined, flow in the region of measuring instruments must be laminar and the velocity distribution must be substantially uniform across the conduit (duct).
Numerous apparatus have been utilized to change turbulent flow into laminar flow, and to obtain a uniform velocity profile from a non-uniform profile. One such apparatus for smoothing the velocity distribution is a converging section in the conduit to raise the average velocity; this, however, can have an adverse affect on turbulence. A more conventional apparatus is a homogeneous resistance element interposed across the conduit which results in the smoothing of the velocity distribution. This can be, for example, a "honeycomb" element with passageways of uniform cross-section and length. Such structures are illustrated, for example, in U.S. Pat. Nos. 3,685,355 and 3,748,901, issued to K. W. DeBraun on Aug. 22, 1972, and July 31, 1973, respectively. This type of structure, in addition, straightens the flow to produce laminar flow rather than turbulent flow. Only a moderate amount of smoothing can be achieved with such homogeneous resistance elements so that often several such elements are employed. This, however, causes a loss of energy and can necessitate extra pumping power to achieve a desired velocity.
A non-homogeneous resistance element is shown and described in U.S. Pat. Nos. 3,842,678 and 3,964,519, issued to K. W. DeBraun, et al., on Oct. 22, 1974, and June 22, 1976, respectively. This non-homogeneous resistance element is also a honeycomb structure having passageways of uniform cross-section but varying in length, with the longest passageways in the center where the velocity is the highest. This structure is accomplished by having a contoured surface of the element facing into the flowing fluid, with the downstream side being flat. This unit can be used alone to flatten the velocity profile; but, preferrably, it is used with a homogeneous resistance element to further smooth the velocity distribution. Velocity measuring instruments, and samplers if desired, are placed proximate but downstream from these resistance elements.
It will be recognized that for a specific system, the unmodified velocity distribution can be first determined, and then a calculation made as to the dimensions of a non-homogeneous resistance element to achieve a given smoothing of the velocity distribution. An element fabricated using those calculations can be made and installed. If, however, the desired smoothing is not sufficiently accomplished, another unit must be fabricated. Even if the element initially performs adequately, changes in the system (new circulator, changes in the duct configuration, etc.) can adversely affect the performance of the resistance element. Thus, a new unit may be required. Even fouling of the surfaces can adversely affect performance.
Accordingly, it is the principal object of the present invention to provide a non-homogeneous resistance element for achieving uniform velocity distribution of fluids, the element adapted for adjustment of the degree of resistance after installation in a fluid circulating system.
It is another object of the present invention to provide a non-homogeneous resistance element for velocity distribution smoothing having a uniform thickness in a direction of fluid flow.
It is still another object of the present invention to provide a non-homogeneous resistance element for adjusting fluid velocities in a conduit, the element having regions of different resistance to flow with that resistance being selectable in each region by varying the equivalent diameter and length of removable resistors.
It is also an object of the present invention to provide a system for producing uniform velocity distribution of air in air circulating systems, the system including at least one non-homogeneous resistance element to achieve such uniform velocity distribution.
It is a further object to provide a non-homogeneous resistance element to achieve uniform velocity distribution in a fluid circulation system, the resistance element being easily constructed.
These and other objects will become apparent upon a consideration of the following descriptive material when taken together with the drawings.